1. Field of the Invention
This invention relates to the in situ formation of an etch mask by using silylating reagents in a lithographic process for device fabrication. The process employs surface-imaging techniques to obtain devices that satisfy fine design rules.
2. Art Background
Design rules for integrated circuit manufacture are becoming increasingly fine. Design rules of 0.5 .mu.m are being replaced by design rules that are less than 0.5 .mu.m. These increasingly fine design rules require processes which can transfer these features into the integrated circuit with concomitant accuracy.
Integrated circuits are manufactured using lithographic processes. A lithographic process employs energy that is introduced onto selected portions of an energy sensitive resist material overlying a substrate. Energy is introduced onto selected portions of the resist through openings in a mask substrate interposed between the energy source and the resist material. These openings in the mask substrate define the pattern. The pattern is transferred into the resist material by the energy that is permitted to pass through the openings in the mask substrate and into the resist. Thus, it is an image of the pattern defined by the mask substrate that is transferred into the resist material.
After the image is transferred into the resist material, the resist material is developed to form a pattern. The pattern is then transferred by etching into the substrate underlying the resist material. Once the pattern is incorporated into the substrate, it becomes a feature of the integrated circuit.
The energy used to expose the resist material, the composition of the resist material, the thickness of the resist material, and many other factors affect the ability of a lithographic process to delineate a feature in a substrate. The smaller the design rule, the more precisely the feature must be delineated.
Another factor which affects the ability of a process to define features in a substrate is the topography of the substrate surface. Substrate surface topography is either planar or non-planar. Non-planar surfaces are referred to as such because their surfaces are not in one single plane. When a resist material is applied over a non-planar substrate, the resist layer only approximately conforms to the non-planar substrate surface. As a result, the distance between the resist surface and the substrate surface tends to be nonuniform. This nonuniformity can adversely affect the pattern developed in a resist material because the depth at which the image is focused in the resist will also vary. If the depth of focus varies over the resist surface, the features resolved in the resist may not satisfy the applicable design rules.
Surface-imaging lithographic processes have been developed which do not require that the resist material be exposed throughout its entire thickness. These processes are referred to as surface-imaging processes because they define features only in the near-surface region of the resist. These surface-imaging resists are particularly useful in lithographic processes which normally define small features over a narrow range of focus. These processes utilize thinner exposed resist layers, have a broader focus range, and ultimately produce more highly resolved features than some processes that use resists that must be exposed throughout their entire thickness. Specifically, surface-imaged resists exhibit increased sensitivity and resolution as the effective resist thickness decreases from 0.5 .mu.m to 0.1 .mu.m depending on the incident radiation and other factors.
A surface-imaging lithographic process which incorporates refractory elements into portions of the resist layer to form an etch mask is described in F. Coopmans, et al. "DESIRE: A New Route to Submicron Optical Lithography", Solid State Technology, pp. 93-99 (June 1987). In the surface-imaging process described in this paper, a silylating reagent is introduced into the resist after it is exposed to radiation. Depending upon the changes in the resist caused by the exposure and subsequent processing, the silylating reagent is incorporated in either the exposed region or the unexposed region of the resist material. During plasma development, the silylating reagent forms an etching mask in the region into which it is incorporated. The silylating reagent, therefore, is used to impart a degree of etching resistance to the resist. The ratio of the etching rate in the region incorporating the reagent to the etching rate in the region intended to be substantially free of reagent is known as the etching selectivity. The region into which the silylating reagent is incorporated theoretically etches more slowly than the region into which the silylating reagent is not incorporated.
The resist material described in Coopmans, et al. is spin-deposited directly over a substrate with a non-planar (stepped) surface. An image is formed in the resist material by subjecting the resist material to a standard ultraviolet (UV) light exposure. The Coopmans', et al. article represents that a silicon-bearing material such as the trimethylsilyl group in hexamethyldisilazane (HMDS) is incorporated into the exposed portions of the Plasmask.RTM. resist material. Plasmask is a registered trademark of Union Chemic Belgique Corp. The article states that the so silylated Plasmask.RTM. is transformed into a silicon-containing image. The image is then incorporated into the substrate using oxygen plasma etching. The article states that the image in the Plasmask.RTM. is transformed into a silicon oxide mask because the silicon in the resist material binds with the active oxygen species and forms a loosely structured protective oxide that stops further etching while the other regions in the resist are etched away by an anisotropic reactive ion etching process. The process results in a negative tone relief image.
The process disclosed in C. Garza, et al., "Manufacturability issues of the DESIRE process", SPIE Advances in Resist Technology and Processing, VI 1086, pp. 229-237 (1989) is similar. The article states that a photoresist, a combination of novolac-resin/diazoquinone-sensitizer, was spin coated onto a substrate. The photoresist was then selectively exposed to energy. The article theorizes that the diazoquinone decomposed in the exposed region and that this brought about chemical changes in the resist that favored the incorporation of HMDS in these regions. Thus, when the resist was exposed to radiation, heated to crosslink unexposed regions, and then treated with a vapor containing HMDS, the trimethylsilyl group was selectively incorporated into the exposed regions of the resist. The resist was then subjected to a dry etch, such as O.sub.2 reactive ion etching. The article states that the etching rate for the area into which HMDS had not been incorporated, was faster than the etching rate for the area in which HMDS had been incorporated. An image corresponding to the negative of the image transferred into the resist was then transferred into the substrate. Garza et al., specify a preference for magnetically enhanced ion etching (MIE) to obtain acceptable resolution and focus latitude. The process described in Garza et al. leaves residues in the unsilylated regions after pattern development which are undesirable.
The surface-imaging process described in C. A. Spence, et al., "Silylation of poly(t-BOC) styrene resists: Performance and Mechanisms", SPIE Advances in Resist Technology and Processing, VII, 1262,, pp. 344-350 (1990). employs a single layer of resist material deposited over a substrate. The resist material so deposited is described as a copolymer of styrene and p-tertbutoxycarbonyloxystyrene. The resist described in the article was subjected to a patterned exposure. Upon heating the tert-butoxycarbonyl groups were cleaved from the resist material in the exposed region and replaced by hydrogen. According to the article, this cleavage produces phenolic sites on the polymer.
A silicon-containing material, HMDS, was introduced into the Spence et at. resist layer after the patterned exposure. The HMDS was preferentially absorbed by the polymer in the exposed region. Spence et at. state that this preferential absorption was caused by the presence of phenolic sites on the resist polymer. Spence et al. reported a negative image resulted when the resist so silylated was etched by oxygen (O.sub.2) reactive ion etching.
M. A. Hartney, et at., "Silylation processes based on ultraviolet laser-induced crosslinking", Journal of Vacuum Science and Technology, B8(6), pp. 1476-1480 (1990) reports that a phenolic resin was spin-deposited onto a substrate. The exposed resin was then subjected to a patterned exposure. The resist material in the exposed region reportedly underwent crosslinking, while the resist material in the unexposed region reportedly did not. The resist material was then silylated. Dimethylsilyldimethylamine (DMSDMA) was used as the silylating reagent. The DMSDMA selectively diffused into the uncrosslinked regions of the resist material, and reacted with it to provide a positive-tone etch mask. The silylated resists were then etched in a parallel plate reactive ion etching system.
The purpose of the in situ etch masks described in the articles enumerated above is to impart etching selectivity between the exposed and unexposed regions of the resist material. The greater the etching selectivity, the better the resolution between the exposed and unexposed regions during development. In some of the processes described in the references that were previously discussed, adequate etch selectivity was not achieved. In Garza et at., residues were observed which indicates inadequate etch selectivity. Therefore, a surface-imaging process that provides better etch selectivity than the processes described above is desired.